Power supply network



t- 1940- w. VAN B. ROBERTS ,925

Pom SUPPLY nz'rwoax Filed Feb. 3, 1938 INVENTOR WALTER VANBKOBERTS ATTORNEY Patented Oct. 22, 1940 UNITED STATES POWER SUPPLY NETWORK Waiter van a Roberts, to Radio Corporation of Delaware Princeton, N. 1.,asaignor of America, a corporation Application February 3, 1938.8eriai No. 188,498

source relatively high as compared to the normal.

operating voltage of the device.

It is an object of the invention to provide an arrangement such that regardless of the resistance of the device, the power input theretocannot exceed a predetermined value.

A further object of the invention is to provide a power supply arrangement of the type described,having the property that, regardless of the resistance of the load device, the power input thereto can never exceed the power input occurring in the normal operating condition. In such a case, the load device may be operated normally with a power input substantially equal to the maximum allowable input without any danger 0! U exceeding this value of input by improper adjustments of the load system resulting in a departure from the normal resistance of the load.

The invention will be described with reference to a power supply system for the screen circuit 85 of a screen grid tube; however, as will be pointed out later, the method is equally applicable to any other type of load.

In order that the advantages of the present in vention may be more fully appreciated, the draw- '0 backs of the prior art methods will be pointed out briefly. To energize the screen of a screen grid tube two methods are generally used. One of these methods provides a fixed voltage supply of the proper value for normal operation, while in 35 the other method there'is utilized a high resistance connected between the screen and the source of plate voltage. In the first method, in addition to the extra equipment required or the large drain of power from the high voltage supply that is do necessary if a potentiometer is'connected across the plate supply that has sufliciently low resistance to provide a substantially constant screen voltage, there is the danger that ii the tube is operated in other than correct adjustment, the

45 screen current may rise to excessive values, so

that the screen power input exceeds the maximum allowable value. For this reason, the users of screen grid tubes are warned, as, for example,

in the instruction sheet for the RCA type 814 0 tube, which says, "If the screen voltage is obtained from a separate source or from a potentiometer, plate voltage should be applied before the screen voltage or simultaneously with it; otherwise, with voltage on the screen only, the

55 screen current may be large enough to cause excessive screen dissipation. If the second method is utilized, that is, if screen voltage is obtained from the plate power supply through a series resistance, there is danger of the full plate voltage reaching the screen, which may not be suiilcient- 1y insulated for such high voltage. Again quotin: from the RCA type 814 tube instruction sheet, with this method, however, it is important that the high voltage supply switch be opened before the filament circuit is opened or the R. F. excitation is removed; otherwise full supply voltage will be placed on the screen." The presentinvention avoids both of the above mentioned dangers.

The single figure of the accompanying drawing illustrates the application of the present invention to a screen grid circuit of a screen grid tube.

In the drawing, the tube i is provided with a? cathode K, an input grid G1, a screen grid G: and an anode A. The energy to be amplified is impressed between input terminals 2 and I of the primary of input transformer T1 and thus fed to the input of the tube which includes the usual tuned circuit comprising condenser C1 shunted across the secondary of transformer T1, grid condenser C: and a direct current path between grid (31 and cathode K comprising choke coil :1 and resistance r in series. The anode circuit of tube 1 includes the tuned circuit comprising the primary of output transformer T: shunted by condenser C4 and a suitable source of anode current represented generally as a battery E. For supplying the screen grid G2 with proper potential two resistors a and b are connected in series across the source E and the grid G: is connected to a point intermediate the two resistors a and b. A by-pass condenser C3 is connected between the grid Ga and the cathode K.

From the above, it is seen that the screen grid tube amplifier circuit has its screen circuit supplied with power from the plate voltage source by way of a current-limiting resistance a, and a voltage-limiting resistance b, whose values are chosen in accordance with the present invention m the manner described hereinafter.

In what follows, .2 denotes the normal screen voltage and i the normal screen current, while E is on plate supply voltage. The value of the resistances a and I; must be so chosen that in normal operation the screen current and voltage havethe values e and 2', respectively. By a simple application of Kirchoiis laws it is easily shown that this requirement leads to the relation between and a given by the equation,

cu m -1 tion,

L b a 1 II From a comparison of Equations I and II it easily can be seen that the individual values ofa and b are as follows:

a= 3. 7 III =ih b (lf)i W I is given by the equation,

E Max. lllpllt='- figg Thus, in order to avoid exceeding the maximum allowable screen input, a value of I may be assumed, and then, by means of Equation V it is possible to determine whether there is any condition of operation under which the screen input exceeds the maximum allowable value. In order to be entirely safe, the value of I should 'be so chosen that the expression in Equation V is less than the allowable screen input.

The expression given in Equation V has an extrernum, with respect to j, which occurs when! is equal to twice the ratio of the normal screen voltage to the plate supply voltage and when 1 is given this value the maximum possible screen input is ei watts. That'is to say, in this particular condition the screen input is both the normal input and also the maximum possible input when the circuit is functioning normally, while, for any other condition. the screen input is reduced. Thus, it is possible to make the normal screen input equal to the maximum allowable input, since there is no danger that the screen input can ever exceed this value under any conditions of adjustment.

Equation V may be readily thrown into another form by making use of Equations I and II to eliminate the quantities j and b from Equation V as follows: I

From Equation II we have Now using Equation I to eliminate b irom the above we obtain Assuming that a is desired to limit a. maximum input to the allowable value W, Equation is solved for a and we obtain To make the application of the invention clear,

consider the 814 type tube previously mentioned for which normal operating conditions include the following: 5 7

E=l250 volts: e=300 volts; i=10 milliamperes.

Taking I as twice the ratio of the screen voltage to the plate supply voltage, as explained above, we have I: .48. From Equations III and IV together with this value of f, the individual values of resistances a and b may be determined. When such resistances are employed, it is impossible by any maladjustment of the circuit, or incorrect order of closing or opening switches, to cause a screen input of more than 3 watts, which value is the normal operating screen input.

While 1 the foregoing adjustment is recommended where the normal input is substantially equal to the maximum allowable input, in many cases the allowable input is greater than the normal input. In the 814 tube the allowable input is 10 watts. Hence, in such a case it is possible to choose a smaller value of ,f, and thus assure an even smaller rise of screen voltage. To determine the smallest value of j that can safely be employed, various values are assumed and the smallest one is chosen, which, when substituted in Equation V, gives a value close to but not exceeding the maximum allowable screen input. The value of I thus determined is then used in Equations III and IV to determine the individual values of resistances a and b.

While the above explanation has confined itself to the case of a power supply for the screen circuit of a screen grid tube, it will be apparent that the same formulae may be used in exactly the same way to design a resistance network for supplying power to a variable resistance load of any nature whatsoever both for direct and alternating current power. The only requirement is that the power supply source must have a voltage at least twice the normal voltage desired across the load, if the system is to be operated in the condition where the normal power input to the load coinoperated at a normal voltage e and a normal current i, and has an allowable dissipation W,

and the voltage E of said source is high compared to said normal load voltage, a first resistance device, a series circuit including said source, said first resistance device and said load; a second resistance device shunted across said load, the resistance a of said first resistance device being given by the equation ith-vat] and the resistance b of said second named resistance device being given by the equation zzaz dleass 02? any vafiati-m m the mic! 1cm devise.

2= In combmatiw, load devise subject to vmmimis from its n-urmal rwstanca vmue ts be opamted sis a normal VQEELEE e a normal cummii, a swarm @f vcltage wmse witage E is relativeiy high b0 said 2102- Mmd vofitage, and means far energizing said fmm said source comprising a. first re5istmme same 0? device, a series circuit including saiwl Somme, fmst msisizance device, and had; & seecond resistance device shunised across the resistame e1 said first resistance device wing whereby Emma current are supplied is said mm dwia: W1. the resistazwe 05 $9M 162361 flevise is @f me: J21 value whifie the zwwer input in s devise reduced Em ether values of RQSiSiSQiECE said 210mm vain-e mm in adfilfilon the veltage supplied is said device never exceed twice the ummal load; vazltage result at any variatian the z'esist-ame cf said Boa-c1 device. 

